The present invention relates to circuit breaker motor operator control circuits, and specifically to improvement over the motor operator control circuit disclosed in U.S. Pat. No. 4,042,896, whose disclosure is specifically incorporated herein by reference.
A common practice in circuit breaker motor operators using permanent magnet DC motors is to simply remove power and then abruptly switch a braking resistor across the motor armature. The motor then becomes a generator whose current output is forced to flow through the braking resistor. Dynamic braking is thus imposed on the motor, and its output shaft is abruptly braked to a stop. This approach, while imminently simple and effective will develop problems if several unfortunately common situations are permitted to occur. If, while the motor is being braked, the control circuit is signalled to re-energize the motor, arcing occurs as relay contacts operate to switch out the braking resistor and to switch in the line voltage across the motor armature. If the arc across the contacts switching out the braking resistor does not clear, line voltage will be imposed across the braking resistor via this persistent arc. Under these circumstances, this arc may never clear and the braking resistor would then soon burn out. This situation simply underlines a basic problem which can develop if the motor is permitted to be re-energized before it has been substantially braked to a stop, even apart from the arcing consideration. If this situation occurs repeatedly, overheating results and, if persistent, both the motor and the braking resistor can be destroyed.
An additional, unrelated problem is engendered if for some reason the breaker operating mechanism fails to become charged after the motor operator has gone through a charging cycle. The typical culprit is an undervoltage release which, because of an internal malfunction or a persistent undervoltage condition, has assumed a condition disabling the breaker latch. If the motor operator control circuit logic is established for automatic recharging of the breaker mechanism, the operator will continuously recycle in repeated, vain attempts to charge the breaker mechanism. This so-called "pumping" situation is extremely hard on the breaker mechanism, not to mention the motor operator itself which is typically not designed for such a demanding duty cycle.
It is accordingly an object of the present invention to provide an improved control circuit for a circuit breaker motor operator mechanism.
An additional object is to provide a circuit breaker motor operator control circuit of the above character which is equipped to automatically prevent indiscriminate re-energization of the motor.
Yet another object is to provide a control circuit of the above character which automatically operates to prevent energization of the motor until it has been substantially braked to a stop.
A further object is to provide a control circuit of the above character which incorporates an anti-pumping feature.
Another object of the present invention is to provide a control circuit of the above character which is inexpensive to manufacture, relatively simple, and yet reliable in operation.
Other objects of the invention will in part be obvious and in part appear hereinafter.